1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor optical device having an optical grating.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 5-67848 discloses a method for manufacturing an optical semiconductor device having an optical grating. In the method for manufacturing an optical semiconductor device, a reactive ion etching method that uses methane gas or ethane gas is used for forming an optical grating.
In non-Patent Document 1 (Kinoshita et al. Etch-Profile Control of Quarter-Micron Resist Pattern Using O2 Supermagnetron Plasma Added with N2 Gas, Technical Report of IEICE, The Institute of Electronics, Information and Communication Engineers, SDM94-114 (1994-10), pp 21-26.), there is described a method for forming a resist pattern for manufacturing an ultra-large-scale integrated circuit (ULSI) formed on a Si semiconductor. Specifically, the method includes the following steps. A two-layer resist composed of a resist layer not containing silicon (Si) having a thickness of 1.2 μm and a resist layer containing silicon (Si) is directly applied on a silicon substrate. The resist layer containing Si is an upper layer disposed on the resist layer not containing silicon (Si). The resist layer containing Si is processed to form a pattern having a narrow pattern width of about 0.5 μm and large thickness of 1.2 μm, or a pattern having a high aspect ratio (the ratio of pattern width to thickness of the resist). Subsequently, the resist layer not containing Si is etched by a plasma etching method that uses oxygen gas mixed with nitrogen gas, using the resist layer containing Si as a mask. Consequently, the pattern formed on the resist layer containing Si is transferred to the resist layer not containing Si, and a pattern having a high aspect ratio is formed. Non-Patent Document 1 also discloses that the resist layer not containing Si is etched while the silicon substrate is cooled.